Aging resistance is required for cold rolled steel sheets used for automobile bodies, electronic appliances, and the like, together with a high strength and formability thereof. The term “aging” refers to a strain aging phenomenon, which causes a defect, what is called “stretcher strain”, caused by hardening occurring when solid solution elements, such as C and N, are fixed to dislocations.
Aging resistance can be imparted upon the cold rolled steel sheets through batch annealing of aluminum-killed steels. However, batch annealing requires an extended annealing time, thereby reducing productivity, and causing severe variation in mechanical properties depending on positions on the steel sheet. Accordingly, interstitial free (IF) steel is mainly used, which is produced by adding intensive carbide or nitride-forming elements, such as Ti or Nb, followed by continuous annealing.
In order to produce the IF steel, the intensive carbide or nitride-forming elements, such as Ti or Nb, must be added. With regard to this, since these elements are likely to raise the recrystallization temperature, the continuous annealing must be performed at a high temperature. As a result, such a process for manufacturing the IF steel causes a decrease in productivity, an increase in manufacturing costs due to large energy consumption, and severe environmental problems. Moreover, the high temperature annealing typically causes various defects, such as cracks, deformation, and the like.
Furthermore, since Ti and Nb have an intensive oxidizing property, these elements generate a great number of non-metallic inclusions, causing surface defects on the steel sheet. Additionally, IF steel has fragile grain boundaries, and is thus subject to, what is so called, “a secondary work embrittlement,” which causes embrittlement of the steel sheet after forming. In order to prevent the secondary work embrittlement, elements including B are added. Meanwhile, in the case where IF steel is used for the products subjected to surface treatments, such as plating, coating and the like, lots of defects typically occur on the surface of the products.
In order to solve the problems, steel without Ti or Nb has been suggested. As an example, Japanese Patent Laid-open Publications No. (Hei) 6-093376, 6-093377, and 6-212354 disclose a method of improving aging resistance of steel sheets by means of strict control of carbon content within a range of 0.0001˜0.0015 wt %, in which B is added in a range of 0.0001˜0.003 wt % instead of Ti or Nb.
According to the above disclosures, since the aging resistance cannot be sufficiently ensured, quenching is needed after annealing the steel in order to ensure the aging resistance. However, in this case, there is a problem in that the quenching is usually performed as a water quench in a water bath, creating an oxidized coat on the steel sheet, and is thus accompanied with pickling in order to remove the oxidized coat, thereby causing the surface defects on the steel sheet, which require additional manufacturing costs. Moreover, the steel sheet has a low strength. Additionally, since the steel sheet has poor in-plane anisotropy, creating wrinkles and ears on the steel sheet, the method suffers from large material consumption.
Meanwhile, the inventors of the present invention have suggested a method of manufacturing cold rolled steel sheets having excellent stretching formability with improved ductility without adding Ti or Nb, disclosed in Korean Patent Laid-open Publication No. 2000-0039137. The method comprises the steps of: hot-rolling a steel slab with finish rolling at an Ar3 transformation temperature or more to provide a hot rolled steel sheet, the steel slab comprising, in terms of weight %: 0.0005˜0.002% of C, 0.05˜0.03% of Mn, 0.015% or less of P, 0.01˜0.08% of Al; 0.001˜0.005% of N; and the balance of Fe and other unavoidable impurities, wherein the composition of C, N, S, and P satisfies the relationship: C+N+S+P≦0.025%; coiling the steel sheet at a temperature of 750° C. or less; cold rolling the wound steel sheet at a reduction rate of 50˜90%; and continuous annealing the cold rolled steel sheet at a temperature of 650˜850° C. for 10 seconds or more. The cold rolled steel sheet manufactured by the method has excellent ductility while ensuring the aging resistance. However, according to the method of the disclosure, since the C content, the N content, the S content, and the P content must be controlled to satisfy the relationship: C+N+S+P≦0.025% in the cold rolled steel sheet, it is necessary to intensify desulphurization capability and dephosphorylation capability during a manufacturing process, thereby causing problems in productivity and manufacturing costs. In view of mechanical properties, since the yield strength of the finally manufactured steel sheet is excessively low, it is necessary to use a relatively thick material. Additionally, upon processing, there is a problem in that due to an excessively high in-plane anisotropy index (Δr), excessive wrinkles are created on the steel sheet, causing fracture of the steel sheet.
The inventors of the present invention have also suggested a method of manufacturing a cold rolled steel sheet, which can improve the yield strength of high strength steel having a 340 MPa grade-tensile strength, disclosed in Korean Patent Laid-open Publication No. 2002-0049667. The method comprises the steps of: hot-rolling a steel slab at an Ar3 transformation temperature or more to provide a hot rolled steel sheet, the steel slab comprising, in terms of weight %: 0.0005˜0.003% of C, 0.1% or less of Mn, 0.003˜0.02% of S, 0.03˜0.07% of P, 0.01˜0.1% of Al, 0.005% or less of N, and 0.05˜0.3% of Cu, wherein the atomic ratio of Cu/S is 2˜10; cold rolling the wound steel sheet at a reduction rate of 50˜90%; and continuous annealing the cold rolled steel sheet at a temperature of 700˜880° C. for 10 seconds to 5 minutes. The cold rolled steel sheet manufactured by the method has an improved yield strength of 240 MPa in a 340 MPa-grade high tensile strength steel. However, since the aging index of the steel sheet is greater than 30 MPa, the aging resistance cannot be ensured for this steel sheet, and since the steel sheet has a high in-plane anisotropy index (Δr) of 0.5 or more at a plasticity-anisotropy index (rm) of 1.8 level, excessive wrinkles are created on the steel sheet, causing the fracture of the steel sheet.
Meanwhile, a cold rolled steel sheet is known in the prior art, which is a high strength cold rolled steel sheet having the aging resistance, and which is manufactured by adding 0.3˜0.7% of Mn and Ti to an extremely low carbon steel while increasing a phosphorus content in the carbon steel. The cold rolled steel sheet has a ductility-brittleness transition temperature of 0˜30° C.; that is, the cold rolled steel sheet has poor secondary work embrittlement to the extent that causes the fracture at a room temperature upon impact.